Rainfall on Titan May Create Propane Aquifers, Study Suggests

Illustration of a cross-section of Titan's surface and near-subsurface, showing the surface lakes/seas, underground aquifers, clathrate layers and icy crust. Image Credit: ESA/ATG medialab

Illustration of a cross-section of Titan’s surface and near-subsurface, showing the surface lakes/seas, underground aquifers, clathrate layers, and icy crust. Image Credit: ESA/ATG medialab

Saturn’s largest moon, Titan, is a very alien yet eerily Earth-like world, with rain, rivers, lakes, and seas; seen from above, the landscape has a familiar look to it. But those lakes, seas, and rivers are fed by a different kind of rainfallliquid methane/ethane. It is far too cold on the surface for liquid water, but the liquid hydrocarbons nicely fill in for H20 in Titan’s “water cycle.” Now, a new study shows how this rainfall interacts with and changes underground aquifers.

The study, led by Olivier Mousis, a Cassini research associate at the University of Franche-Comté in France, suggests that runoff from rainfall is chemically altered by icy materials called clathrates within the aquifers below the surface. These aquifers, composed of propane and ethane, then feed some of the lakes and seas on the surface, although the rainfall itself is thought to initially provide the liquid in most of them.

Titan is a bizarre world of methane seas, lakes, rivers and rain on its surface and a salty water ocean below. Image Credit: NASA/JPL-Caltech

Titan is a bizarre world of methane seas, lakes, rivers, and rain on its surface and a salty water ocean below. Image Credit: NASA/JPL-Caltech

“We knew that a significant fraction of the lakes on Titan’s surface might possibly be connected with hidden bodies of liquid beneath Titan’s crust, but we just didn’t know how they would interact,” said Mousis. “Now, we have a better idea of what these hidden lakes or oceans could be like.”

The study modelled how an aquifer, or reservoir, of hydrocarbon liquids would spread through Titan’s icy upper crust. The result was that a secondary reservoir, composed of clathrates, would slowly form below the original one which had been formed by rainfall.

According to Mathieu Choukroun of JPL, one of three co-authors of the study, “Our study shows that the composition of Titan’s underground liquid reservoirs can change significantly through their interaction with the icy subsurface, provided the reservoirs are cut off from the atmosphere for some period of time.”

Radar image from Cassini of Ligeia Mare, the second largest sea on Titan, with associated rivers. Image Credit: NASA/JPL-Caltech/ASI/Cornell

Radar image from Cassini of Ligeia Mare, the second largest sea on Titan, with associated rivers. Image Credit: NASA/JPL-Caltech/ASI/Cornell

How would the transformation occur? Through a process called fractionation, clathrates can split molecules into both solid and liquid phases; beneath the surface, the original clathrate reservoirs would fractionate the liquid methane, slowly changing its composition to propane or ethane.

Basically, there could be two different types of lakes/seas and rivers on Titan: those fed by underground springs would be composed of propane or ethane, while others formed from rainfall would still be composed primarily of methane.

The results provide important clues as to how the “methane cycle” on Titan works, and how it is both similar to and different from the hydrological cycle on Earth. It is worth noting also that Titan is thought to have a liquid water ocean deeper down below the surface, where it is warmer. It is unlikely that any of that water would ever make it to the surface, but like with other moons, such as Europa and Enceladus, the presence of liquid water raises the prospect of possible life, at least microscopic. Some scientists even think that life could be possible in the methane lakes and seas, but it would have to be well-adapted somehow to that liquid yet freezing cold environment, if it ever originated to begin with.

The new study was published in the Sept. 1, 2014, issue of the journal Icarus.

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